Nearly all communications cables now being installed in domestic communications systems are buried beneath the ground. Such buried cables include not only cables composed of insulated electrical conductors such as insulated copper wire, but also optical waveguide cables composed of glass or other materials. Most of these cables are waterproofed because of the antagonistic environment factors present underground, water being the chief concern.
U.S. Pat. No. 4,176,240 reports that attempts to waterproof buried cable began nearly one-hundred years ago, but were not successful in the practical sense until the production of plastic insulated cable (PIC) during the 1950's. PIC cables with dual plastic coatings have been reported successfully buried in dry environments. Up to 1950, it was a general practice in buried cable installations, where water was a problem, to pressurize the interior of the cable. This practice was successful in excluding water from the cable interior; however, pressurized cable is expensive to maintain and for this reason has fallen from general use. Unpressurized PIC cables fail to solve the water problem because water migrates through the plastic jacket into the interior of the cable and disrupts or deteriorates communication service. Water can permeate a PIC sheath through a localized opening and thereafter is free to follow any channel within the cable as far as physical forces will allow, often hundreds of feet, to accumulate and flood a localized cable segment. This water not only upsets the capacitance balance of electrical transmission lines, but also introduces potential corrosion which, after an extended time, tends to deteriorate the useful life of the water soaked transmission medium. Water flooding a cable containing optical waveguides can be deleterious to optical transmission.
One solution widely adopted in an attempt to solve the problem of water in a communications cable is to fill the interior of the cable, not otherwise occupied by the conductors, with an insoluble filling material that would have the propensity to plug the cable and stop the channeling of water and thus flooding of a particular segment. It has been said many times and recently restated in U.S. Pat. No. 4,176,240, the physical function of filling a cable with a filling material is straight forward, the selection of the nature and kind of filler material is not. One must consider the hydrophobic nature of the materials used, stability on aging, low temperature properties, flow characteristics at elevated temperatures, processing characteristics, handling characteristics, dielectric properties, shrinkage, toxicity and cost, just to name the important ones.
The structure of electrical communications cables, such as telephone cables, involves assembly of twisted pairs of insulated conductors in a core surrounded by a helically wound or longitudinally applied heat barrier wrap, with or without a metallic shield or armor and finally a polymeric jacket. Cables containing optical waveguides are generally smaller in comparison to their twisted pair cousins, but have a similar structure.
Water, in the vapor or liquid form, may enter and displace air inside unfilled cables by the process of diffusion pumping through the polymeric jacket, or by migration through open cable ends, leaky splices or flooded terminals, or through manufacturing defects and other openings in the protective materials resulting from lightening strike, mechanical damage or rodent attack. This water increases the transmission loss of electrical or light signals and thus severely degrades the performance of communications cables. In addition, corrosive effects of water on metal components may result in an open electrical circuit.
As a remedy, the prior art teaches the filling of the interstitial space within a cable with waterproofing or so-called filling material. These materials are usually greaselike compositions and by the virtue of their physical presence in a cable, they effectively block the entry and migration of water. However, even grease filled cables have inherent problems. In filled electrical communications cable, thicker insulation is required to obtain the same mutual capacitance of the pairs of conductors as in unfilled or air-core type cables. As a result, the cables become larger, heavier and more expensive. Due to the greasy nature of the filling materials, handling becomes more involved for filled electrical and for filled light waveguide cables, resulting in reduced splicing efficiency. Also, flow characteristics of the filling materials at elevated temperatures are such that they pose maintenance difficulties in aerial installations. Moreover, the filled cables are heavier and they become stiff in cold climates and are difficult to install.
The state of the prior art relevant to the instant invention is best described by U.S. Pat. Nos. 3,607,487; 3,717,716; 3,683,104; 3,843,568; 3,879,575; 4,176,240 and the applicant's co-pending patent application Ser. No. 146,339, filed May 2, 1980, which is a continuation-in-part of application Ser. No. 106,866, filed Dec. 26, 1979, now abandoned.
U.S. Pat. No. 3,683,104, issued in 1972, and U.S. Pat. No. 3,843,568, issued in 1974, disclose heat-resistant mixtures of petrolatum and partially crosslinked polymers to which hollow synthetic thermoplastic particles prepared from a copolymer of styrene and acrylonitrile are added.
U.S. Pat. No. 3,879,575, issued in 1975, disclosed waterproofing compounds for protecting electrical conductor splice points. The compunds are blends of from 84.5 to 92.5 parts by weight of mineral oil, from 0.5 to 3.0 parts by weight of styrene-isoprene-styrene block copolymer, and from 6.0 to 13.0 parts by weight of polyethylene having a weight average molecular weight above 2000.
Materials that exhibit some of the properties desirable in a filling mixture and which have been used widely in the communications industry are described in U.S. Pat. Nos. 3,607,487 and 3,717,716 which disclose a petroleum jelly, mixed with a polymer (usually polyethylene) to impart consistency and prevent flowing in warm temperatures. Additionally, in U.S. Pat. No. 4,176,240 a cable filler is disclosed which is composed of a styrene-ethylene-butylene-styrene block copolymer dissolved in ASTM Type 104A (naphthenic) mineral oil with polyethylene added for consistency. This filler is represented to be an improvement over petroleum jelly or petrolatum compounds of the prior art. In Applicant's co-pending application Ser. No. 146,339, filed May 2, 1980, entitled "Filler Materials for Communications Cables," filling materials are disclosed which use petrolatum as a base, and contain either polyethylene or glycerol hydroxy stearate as preferred embodiments and glass or ceramic type microspheres.